1. Field of the Invention
The present invention relates to a radiation image recording and reproducing method, and more particularly, to a radiation image recording and reproducing method utilizing a divalent europium activated barium fluorohalide stimulable phosphor.
2. Description of Prior Art
For obtaining a radiation image, there has been conventionally employed a radiography utilizing a combination of a radiographic film having an emulsion layer containing a photosensitive silver salt material and an intensifying screen. As a method replacing the above-mentioned conventional radiography, a radiation image recording and reproducing method utilizing a stimulable phosphor stated, for instance, in U.S. Pat. No. 4,239,968, has been recently paid much attention. The radiation image recording and reproducing method involves steps of causing the stimulable phosphor to absorb a radiation having passed through an object or having radiated from an object; sequentially exciting (or scanning) the phosphor with an electromagnetic wave such as visible light or infrared rays (stimulating rays) to release the radiation energy stored in the phosphor as light emission (stimulated emission); photoelectrically detecting the emitted light to obtain electric signals; and reproducing the radiation image of the object as a visible image from the electric signals.
In the radiation image recording and reproducing method, a radiation image is obtainable with a sufficient amount of information by applying a radiation to the object at a considerably smaller dose as compared with the conventional radiography. Accordingly, the radiation image recording and reproducing method is of great value, especially when the method is used for medical diagnosis.
A representative example of the stimulable phosphor employable in the radiation image recording and reproducing method is a divalent europium activated barium fluorohalide phosphor (BaFX:Eu.sup.2+, in which X is at least one element selected from the group consisting of Br, Cl and I), as disclosed in the above-mentioned U.S. Pat. No. 4,239,968. This phosphor gives light emission of high intensity (stimulated emission) and the peak wavelength of the emission is approximately 390 nm, and hence the phosphor is practically very useful. It is also known that the stimulation spectrum of the phosphor shows the maximum emission intensity at a stimulation wavelength of approx. 600 nm. Accordingly, in practicing the radiation image recording and reproducing method utilizing said phosphor, it is proposed that stimulating rays having a wavelength in the vicinity of the peak wavelength in the stimulation spectrum thereof, namely of approx. 600 nm, be employed to enhance the sensitivity of the method, whereby reducing the exposure dose given to the object. For instance, an He-Ne laser beam (wavelength: 633 nm) is described as an example of stimulating rays in the above-mentioned U.S. Pat. No. 4,239,968.
In the radiation image recording and reproducing method, a stimulable phosphor is generally employed in the form of a radiation image storage panel containing it. The radiation image storage panel comprises a support and a phosphor layer containing a stimulable phosphor which is provided on one surface of the support.
It is noted that a radiation image storage panel having stood for a long period of time is apt to cause decrease of quality of an image. There is considered that such decrease occurrs because the stimulable phosphor of the panel absorbs radiations radiating from radioisotopes such as .sup.226 Ra and .sup.40 K which is present in the phosphor in a very small amount or environmental radiations and the radiation energy stored therein becomes noises in the resulting image.
The radiation image storage panel employed in the method hardly deteriorates upon exposure to a radiation or to stimulating rays, so that the panel can be used repeatedly for a long period. When the radiation energy stored in the panel as a latent image through exposing the panel to a radiation is released as stimulated emission upon excitation thereof with stimulating rays, only a portion of the stored radiation energy is released and another portion of the radiation energy generally still remains in the panel. Accordingly, the remaining energy causes a noise in the subsequent use of the panel. The noise caused by the radiation energy remaining in the panel has a tendency to markedly appear particularly in the case that a small dose of radiation is applied to the panel in the subsequent use thereof.
For removing (erasing) the radiation energy which causes the noise, it is proposed in U.S. Pat. No. 4,400,619 that light having a wavelength in the stimulation wavelength region of the phosphor is applied to the radiation image storage panel in advance of applying a radiation such as X-rays to the panel in the subsequent use.
Therefore, the stimulable phosphor employed in the above-mentioned radiation image recording and reproducing method is desired not only to give stimulated emission of high intensity but also to show a high efficiency of erasure. In other words, desirable is such a phosphor that difference between the amount of stimulated emission given upon excitation with stimulating rays after exposure to a radiation and the amount of stimulated emission given upon excitation with the stimulating rays after applying the erasing operation thereto is as large as possible.